(verb) To give more information than is necessary (or appropriate) during a conversation--usually in a group setting. Although men are capable of the occasional "overshare," women are the most common offenders of this faux-pas--especially while on first dates with aloof douche bags.
Ladies, if you're on a date with a guy you've just met, don't "overshare." He doesn't want (or need) to hear about your valiant battle to overcome ANY of the following: eating disorder/catastrophic mental or physical health issue (of you or anyone in your immediate family)/rape/drug and/or alcohol and/or sex and/or shopping and/or gambling addiction/death of any member of your family (including pets). If you have no interest in seeing him ever again and want the date to end as quickly as possible, then by all means, share away!
(Note: if you are one of those people who likes to say "Ew, t.m.i.," whenever anyone reveals anything remotely personal or interesting or honest in public, fuck off, you're common and shallow and should be smothered in your sleep.)
I am the person who first thought of this word and would like for it someday replace the phrase, "Too Much Information" which is a rude and prissy expression, even if you shorten it to T.M.I. I think "overshare," is far less judgmental and insulting way to tell someone they need to ratchet it back a few notches...anyway, let me know what you think!