mirth

Mirth comes from Old English myrgþ, meaning "mirth, joy," built from merry plus the abstract-noun suffix -th, tracing back to Proto-West Germanic *murgiþu, which meant "briefness, brevity."

The root idea was about shortness — how quickly something passes — but by Old English times it had already settled into meaning joy and gladness, the sense it still carries today.

It shares a root with merry, since mirth is literally that word plus a suffix turning a feeling into a noun; it's also cognate with Middle Dutch merchte, meaning pleasure or delight.