Old English widewe, wuduwe, from Proto-Germanic *widuwō, from PIE adjective *widhewo (source also of Sanskrit vidhuh "lonely, solitary," vidhava "widow;" Avestan vithava, Latin vidua, Old Church Slavonic vidova, Russian vdova, Old Irish fedb, Welsh guedeu "widow;" Persian beva, Greek eitheos "unmarried man;" Latin viduus "bereft, void"), considered to be from root *uidh- "to separate, divide" (see with).
Extended to "woman separated from or deserted by her husband" from mid-15c. (usually in a combination, such as widow bewitched, grass widow). As a prefix to a name, attested from 1570s.
The meaning "short line of type" (especially at the top of a column) is implied by 1904 print shop slang. Widow's mite is from Mark xii.43. Widow's peak is from the belief that hair growing to a point on the forehead is an omen of early widowhood, suggestive of the "peak" of a widow's hood.
The widow bird (1747) so-called in reference to the long black tail feathers of the males, suggestive of widows' veils.
The PIE adjective also is considered to be the source of Sanskrit vidhuh "lonely, solitary," vidhava "widow;" Avestan vithava, Latin vidua, Old Church Slavonic vidova, Russian vdova, Old Irish fedb, Welsh guedeu "widow;" Persian beva, Greek eitheos "unmarried man;" Latin viduus "bereft, void." Germanic cognates of the English noun include Old Saxon widowa, Old Frisian widwe, Middle Dutch, Dutch weduwe, Dutch weeuw, Old High German wituwa, German Witwe, Gothic widuwo.