What does lacus mean in Latin?

lacus, a noun, means “a lake, pond, basin; reservoir; a tank, tub, sink, vat, wine-vat”.

What is the genitive of lacus?

The genitive singular is lacūs. lacus is masculine, fourth declension.

How do you pronounce lacus?

In classical Latin, [ˈɫa.kʊs]; in ecclesiastical (church) Latin, [ˈlaː.kus].

lacus, lacūs — meaning & declension

Meaning

masculinea lake, pond, basin; reservoir
masculinea tank, tub, sink, vat, wine-vat
inque cavōs ierant tertia musta lacūsand three times had the must been poured into the hollow wine-vats (The Latin word lacus, meaning ‘‘hollow’’ or ‘‘lake’’, also described a container used in ancient winemaking. See also mustum and must.)8 CE, Ovidius, Fasti 3.558

Latīnē defined in Latin in our graded readers

magna aqua stāns, quam terra cingit.

lacus -ūs m — in Mille Verba, Rūs et Agrīcultūra

magna aqua quae nōn fluit

lake

lacus -ūs (m) — in De Bello Gallico I, ch. 2

aqua magna terrā cīncta

lacūs -ūs (m) — in Metamorphoses I, †

Choosing the right word Döderlein's Latin Synonyms

Lacuna · Lacus · Stagnum · Palus · Uligo · Lama · Lustrum

Lacuna denotes, in poetical language, any standing water, from a sea to a pool; lacus and stagnum are collections of standing water kept sound and fresh by their own springs, or by ebbing and flowing; lacus (liquere) is large enough to bring to mind the image of the open sea, in opp. to the main sea, like λίμνη; stagnum, like a pond, not so large as to resemble a lake, in opp. to a stream, like τέναγος; whereas palus and uligo are collections of standing water corrupted and grown foul; palus (πλυδᾶν) is, like a marsh, a district covered with a surface of foul water, like ἕλος; uligo (from ὀλός) like a moor, a district soaked through with foul water. The palus appears as a mass of water made thick by mud and bog-earth, in which a person may be drowned; uligo only as ground thoroughly soaked with water, in which a man may sink down. Lastly, lamæ and lustra denote standing waters of small extent; lama, a mere dirty and filthy puddle on a high road; lustra, an ill-smelling and noisome quagmire in woods, etc. (v. 30.)

Declension

declension
form	singular	plural
nominative	lacus	lacūs lacora
genitive	lacūs	lacuum
dative	lacuī	lacubus lacibus
accusative	lacum	lacūs lacora
ablative	lacū	lacubus lacibus
vocative	lacus	lacūs

lăcus, ūs (I. gen. laci, Vulg. Dan. 6, 17; 24; id. Jer. 37, 15; Cassiod. Var. 11, 14; dat. and abl. plur. lacis, Anthol. Lat. 5, 71, 10: lacibus, Plin. 23, 1, 20, § 33; usually lacubus, Ov. M. 12, 278 al.), m. root lak, to tear; Gr. λάκος, λακερός, λάκκος; Lat. lacer, lacinia, lacuna, lāma; cf. lacerna; originally any thing hollow, hence. 
I.  A large vessel for liquids, a basin, tank, tub; esp. a vat into which the wine flowed from the press, Cato, R. R. 25; 67, 2; Col. 12, 18, 3: tu quoque devotos, Bacche, relinque lacus, Tib. 2, 3, 64: de lacubus proxima musta tuis, Ov. F. 4, 888; a tank of water, in which heated metal was cooled: alii stridentia tingunt Aera lacu, Verg. G. 4, 173: gelido ceu quondam lamina candens tincta lacu, stridit, Ov. M. 9, 170: ferrum, igne rubens ... lacubus demittit, id. ib. 12, 278.—Hence, 
B.  Transf.: oratio quasi de musto ac lacu fervida, i. e. still new, that has not done fermenting, Cic. Brut. 83, 288.—
II.  A large body of water which rises and falls (opp. stagnum, a standing pool), a lake, pond: agri, aedificia, lacus, stagna, Cic. Agr. 3, 2, 7: exhalant lacus nebulam, Lucr. 5, 463: deae, quae illos Hennenses lacus lucosque incolitis, Cic. Verr. 2, 5, 72, § 188; cf. 2, 4, 48, § 107: Averni, Lucr. 6, 746; Cic. Tusc. 1, 16, 37: Albanus, id. Div. 1, 44, 100: Fucinus, Plin. 36, 15, 24, § 124: dicebar sicco vilior esse lacu, Prop. 2, 14 (3, 6), 12: ad spurcos lacus, Juv. 6, 603.—Poet., of a river: deinde lacu fluvius se condidit alto Ima petens, Verg. A. 8, 66; cf. v. 74; of the Styx, id. ib. 6, 134; 238; 393.—
III.  A large reservoir for water, a basin, tank, cistern (of which there were a great number in Rome), Front. 3; 78; Liv. 39, 44; Plin. 36, 15, 24, § 121: a furno redeuntes lacuque, Hor. S. 1, 4, 37.—A place called Lacus: garruli et malevoli supra Lacum, at the pond (perh. Lacus Curtius or Lacus Juturnae), Plaut. Curc. 4, 1, 16.—Prov.: siccus lacus, for something useless, Prop. 2, 11, 11 (3, 6, 12).—
IV.  
a.  A hole in which lime is slacked, a lime-hole, Vitr. 7, 2, 2.—
b.  One of the bins or receptacles for pulse in a granary: sed et lacubus distinguuntur granaria, ut separatim quaeque legumina ponantur, Col. 1, 6, 14.—
c.  A den or cave for lions: labitur in lacum leonum, Prud. Cath. 4, 65; Vulg. Dan. 6, 7.—
d.  The pit, the place of the dead (cf. II. fin. supra): salvasti me a descendentibus in lacum, Vulg. Psa. 29, 4.—
V.  For lacunar, a panel in a ceiling (ante-class.): resultant aedesque lacusque, Lucil. ap. Serv. Verg. A. 1, 726.

Origins

According to De Vaan, from Proto-Italic *lakus, from Proto-Indo-European *lókus (“lake, pool”); the reason for the change to *a is debated, but perhaps from velarized *[ɫ]. Regarding this proposed phonological development, Schrijver notes that the form lacus may be the only positive evidence for such a sound law, rendering the existence of the rule uncertain. Cognate with Ancient Greek λάκκος (lákkos, “cistern, tank, pit”) and Old English lagu (“sea, ocean, flood”). More at lay.

Descendants

Catalanllac
Frenchlacfrom Old French lac
Portugueselagofrom Old Galician-Portuguese lago
Spanishlagofrom Old Spanish lago
Italianlago
Occitanlac
Romanianlac
Sardinianlagu
Aragoneselaco
Aromanianlac

In English words that came into English from lacus

lacustrinelacustris
cunettelacūna

In the Academy Smith's classical dictionaries

AntiquitiesLacusLacus 1. See FONS 2. See TORCULAR 3. Lacus (βαφὴ) was also used for the bath in which the smith (χαλκεὺς or faber ferrarius) plunged the hot iron to g…