I apologize in advance for romaji. My phone doesn't have a Japanese keyboard.
"atsui" means hot, and there are different kanji for the adjective when referencing different things. "usoi" is not a word to my knowledge, but "usui" means thin.
I read on hi native that うすい is for thin as in between two spaces like a book (slim) while ほそい for a thin thread or tube like a cable.
Things that use あつい can use うすい and things that use ほそい can use ふとい
Feel free to correct me if I'm wrong though!