Billow comes from Old Norse bylgja ('a wave, billow'), from Proto-Germanic *bulgijǭ, from a root meaning 'to swell'. Entered English in the 16th century. The historical trail runs through bylgja in Old Norse and *bulgijǭ in Proto-Germanic. Used as a noun and verb, billow now means "a large rolling wave; to swell or surge outward", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in derived forms such as billowy and billowing. The result is a borrowed or transmitted form whose modern spelling carries a compact record of its route into English.