"barrier across a stream of water to obstruct its flow and raise its level," c. 1400 (early 13c. in surnames), probably from Old Norse dammr or Middle Dutch dam, both from Proto-Germanic *dammaz (source also of Old Frisian damm, German Damm), which is of unknown origin. Also perhaps in part from or reinforced by Old English verb fordemman "to stop up, block."
"animal mother, female parent of a quadruped," mid-15c., damme, variant of dame "lady, mother" (q.v.), which is attested from early 14c. in this secondary sense. The two forms were somewhat interchangeable in Middle English, but the meanings diverged into separate spellings by 16c., and any use of dam for women since then has been slighting.
"obstruct or restrain a flow by means of a dam," c. 1400, from dam (n.1). Related: Dammed; damming.