In describing how I reproduced an address error in replying to an e-mail sent to me as part of a group address, I explained that I highlit the message received, smushed "reply to all" and wrote my response above the header. Immediately I heard back from one of the recipients that I misused the word "highlit". If "highlit" is not the past participle of "to highlight" then what is? And why?

I would probably say highlighted, but that's lousy, too...

I would go for highlighted as well. Steven Pinker has a lot to say about this, but as most of his examples were taken from baseball, I may have misunderstood.

Bingley

My dictionary told me it's highlighted. It's an English-Chinese dictionary, though.

I’m learning English. If u find anywhere I can improve my composition, Pls do let me know. Bow.

Google for "highlit" = 5,430 English pages.

I get 5,840 for highlit. Highlighted gets 27,100,000.

I think Pinker's point was that verbs with recently derived meanings, e.g. a batter flying out, and newly built verbs, e.g. highlight, are regular even if the root verb is irregular.

AHD lists only the regular past:

http://www.bartleby.com/61/70/H0197000.html

If "highlit" is not the past participle of "to highlight" then what is? And why?

Your use of "highlit" assumes that "highlight", because it contains the root word "light", can and should be treated as though it is the same as the word "light" standing alone.

But the "light" in the word "highlight" is used metaphorically, and the 'lighting' is done, not with electricity or combustion, but with a "highlighter" which deposits a yellow smear, or smear of another color, which 'highlights' selected text.

Thus, "highlight" is a new word which is not bound by the variations appropriate to the word "light" standing alone.

"Highlighted" is used in preference to "highlit", not only because it sounds more fluid [please excuse the pun], but also because "lit" places undue emphasis on the literal meaning of the root word "light".

Of course, there is no absolute right or wrong to this, Father Steve. The general prejudice in favor of "highlighted" is intuitive, not canonical.