dam (n.1) Look up dam at Dictionary.com
"water barrier," early 14c., probably from Old Norse dammr or Middle Dutch dam, both from Proto-Germanic *dammaz (cognates: Old Frisian damm, German Damm), of unknown origin.
dam (n.2) Look up dam at Dictionary.com
"animal mother," c.1300, variant of dame (q.v.), also originally used, like that word, for "lady, mother;" but meanings diverged into separate spellings by 16c.
dam (v.) Look up dam at Dictionary.com
late 15c., from dam (n.1). Related: Dammed; damming.