alguien
/ˈalɡjen/ · A1 · pron
someone, somebody
algu- (some, as in algo, alguno) + -ien (from quien, who) = some-who
From Latin aliquem, some person, reshaped in Spanish to rhyme with quien (who). Same algu- as algo (something) and alguno (some). Its opposite nadie (nobody) pairs with it the way nada pairs with algo.