Widower \WID-oh-er\ , noun;
1. A man who has lost his wife by death and has not remarried
Even though English does not use grammatical gender, there are plenty of words that have two versions to denote male or female: waiter/waitress, actor/actress, etc. This is usually done by adding a suffix to the masculine version to indicate female. According to Dictionary.com there is just one instance where the reverse happens: widow/widower.
Widower dates to the mid-14th century as an extension of widow. Previously, the Old English masculine form of widow was widewa and the feminine was widewe. Both derive from Proto-Germanic *widewo from Proto-Indo-European *widhewo, which comes from the base *weidh- ("to separate").