merry == brief?

Wiktionary's etymology:

From Middle English mery, merie, mirie, myrie, murie, murȝe, from Old English meriġe, miriġe, myriġe, myreġe, myrġe (“pleasing, agreeable; pleasant, sweet, delightful; melodious”), from Proto-West Germanic *murgī (“short, slow, leisurely”), from Proto-Germanic *murguz (“short, slow”), from Proto-Indo-European *mréǵʰus (“short”). Cognate with Scots mery, mirry (“merry”), Middle Dutch mergelijc (“pleasant, agreeable, joyful”), Norwegian dialectal myrjel (“small object, figurine”), Latin brevis (“short, small, narrow, shallow”), Ancient Greek βραχύς (brakhús, “short”). Doublet of brief.

The shift from "slow, leisurely" to "agreeable" is an easy one. And likewise the shift from the cause of happiness to the state of happiness — from the OED's sense I.1.a "Of an occupation, event, state, or condition: causing pleasure or happiness; pleasing, delightful" to sense II.4.1 "Full of animated enjoyment (in early use chiefly with reference to feasting or sporting); full of laughter or cheerfulness; joyous".

But the earlier shift from "short" to "slow" is less intuitive: short→long does match slow→fast, but matching brief with slow, not so much. There's a diminutive in the song  "(Have Yourself) A Merry Little Christmas", but "Have yourself a brief little Christmas"? I don't think so.

So have a merry Christmas, of the calendrically designated duration!

N.B. According to genius.com,

“Have Yourself a Merry Little Christmas” is a song written in 1943 by Hugh Martin and Ralph Blane and introduced by Judy Garland in the 1944 MGM musical Meet Me in St. Louis. Frank Sinatra later recorded a version with modified lyrics. In 2007, ASCAP ranked it the third most performed Christmas song during the preceding five years that had been written by ASCAP members. In 2004 it finished at No. 76 in AFI’s 100 Years…100 Songs rankings of the top tunes in American cinema.

The 1944 Judy Garland version is here, and the original verging-on-tragic lyrics are here.