Yay Definition

yā

interjection

Used as an exclamation of pleasure, approval, elation, or victory.

American Heritage

Webster's New World

(colloquial) An expression of happiness.

Yay! I have finally finished my work!

Wiktionary

(colloquial) Alternative form of yea (yes).

Wiktionary

adverb

So; by this amount.

“The fish was yay big” he said, spreading his arms three feet apart.

American Heritage

Webster's New World

This; so.

Yay big, yay high.

Webster's New World

(chiefly US) So, this (accompanied by a hand gesture)

The pony was yay high.

Wiktionary

noun

Webster's New World

The name of the letter for the y sound in Pitman shorthand.

Wiktionary

(US, slang) Cocaine.

Wiktionary

A branch of the Thai language.

Wiktionary

Origin of Yay

Alteration of yea (“yes; even, truly, verily"). More at yea.

From Wiktionary
From Spanish llello.

From Wiktionary
Alteration of yea

From American Heritage Dictionary of the English Language, 5th Edition
Probably from yea

From American Heritage Dictionary of the English Language, 5th Edition