interjection
poor thing; what a pity; poor you
Internet slang derived from 可哀想 (かわいそう). Used online to express sympathy or pity, often with a playful or exaggerated tone.
See also: 可哀想
テストで0点取ったの？カワイソス。
You got a zero on the test? Poor you.
雨でピクニックが中止になった。カワイソス。
The picnic was canceled because of rain. What a pity.