"now and then, at times but not all times," 1520s, from sometime + adverbial genitive -s.
Entries linking to sometimes
sometime adv., adj.
late Old English, sumtime, "at one time or another;" see some + time (n.). The sense of "at an indefinite future time" is from late 14c. As an adjective, "former, late, at one time, for a certain time in the past," late 15c. Also in Middle English sumtide (early 14c.). Compare sometimes.