genu

Genu, genus, n., fourth declension, and one of only a handful of neuter nouns in that declension: cornu and veru are the others you will actually meet. The singular is short and blunt - genu for nominative, accusative and ablative alike - and the plural gives genua, genuum, genibus. Classical Latin prefers genibus for the dative and ablative plural, though genubus is attested and does no harm.

The genitive singular genus is spelled exactly like genus, generis, "kind, race", which is a nuisance only until you notice that context settles it instantly: knees do not have descendants. Flexis genibus, "with knees bent", is the standard phrase for kneeling in liturgical Latin, and genuflect is the same two words carried into English whole.