Whilom

Whilom \HWAHY-luhm\ , adjective;
1. Former; erstwhile
adverb;
1. At one time

According to Etymonline.com:
The definition "at time past" is considered archaic and dates to the 1200's. It comes from Old English hwilum ("at times"), which is the dative case of while.

Dative case of while...scary, right? Anyone who has taken German or Russian or Latin knows about grammatical cases and how tricky they can be for Modern English speakers, but that wasn't always the case (pardon the pun...). Old English was fully inflected with five grammatical cases (nominative, accusative, genitive, dative, and instrumental), and also assigned masculine and feminine gender to all nouns. This really isn't surprising considering how closely related English and German are, so the interesting question is: why don't we use cases and gender anymore?

The short answer is: who knows? One theory is that in areas where Old English speakers heavily intermingled with Old Norse speakers, the use of cases started to decline and eventually disappeared. This could also explain why some of the modern languages that came from Old Norse also don't use case (Danish, Swedish), although some still do (Icelandic, Faroese, Norwegian). The loss of grammatical gender is more puzzling because it may be linked to the influence of Old Norse as well, but all modern Norse-derived languages still use gender. Why English would lose gender with case is uncertain, but the two are probably linked, whether the Old Norse theory is correct or not.
Remnants of our case system do still exist, but almost exclusively in our pronouns. Consider: "I want a soda, please give it to me." Both I and me refer to the first person singular, but they have different forms. In Old English, iċ was nominative and mē was accusative and dative, and our current usage reflects those now-outdated uses.
Grammatical gender still exists as well, but in very weak forms. We don't use it at all in our definite and indefinite articles, unlike some other modern European languages. His/her/it and their various forms reflect gender (and case), but in a very straightforward way that is not really in line with true grammatical gender. In colloquial language things like boats and cars can be referred to as she/her, which definitely mimics grammatical gender, and other word pairings act in a similar way (consider: brother/sister, doe/buck, waiter/waitress).

*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Thursday, November 11