that was delicious
Jocular interjection used as a pun on うまかった (past form of うまい, 'delicious'). The kanji 牛負けた literally means 'the cow lost', creating a humorous ateji wordplay. Not a standard expression; used playfully among friends or in casual online contexts.
See also: うまい
This ramen was delicious! (pun on うまかった)
It says 'ushi maketa', but basically it means 'umakatta' (it was delicious), right?
A jocular ateji pun: the kanji 牛 (cow) and 負けた (lost) are used phonetically to represent うまかった, the past form of うまい (delicious). The phrase literally reads 'the cow lost' but is meant to be read as 'it was delicious'. This is a modern wordplay, not a historical derivation.