From Old English (before 12th century), from Proto-Indo-European '*ǵnéw-' ("knee"), from PIE *ǵnéw- ("knee").
To rest on one's knee or knees; to go down on one's knees as a sign of respect, submission, or prayer.
From Old English 'cnēowlian' (to kneel), from 'cnēow' (knee), from Proto-Germanic '*knewą' (knee), from PIE root *ǵnéw- (knee). The PIE root is remarkably well-preserved across the family: Latin 'genū' (knee), Greek 'góny' (γόνυ, knee), Sanskrit 'jānu' (knee). The initial 'kn-' cluster was pronounced in Old English — 'cnēow' was 'k-NOW' — and the 'k' wasn't silenced
'Knee,' 'kneel,' Latin 'genū,' and Greek 'góny' all descend from the same PIE word *ǵnéw-. This makes 'genuflect' (from Latin 'genū' + 'flectere') and 'kneel' (from Germanic *knewą) etymological cousins saying the same thing in different branches of the family — both literally mean 'to bend the knee.'