The woman accusing Auston Matthews of dropping his pants in front of her was LIVID over the whole situation ... saying of the NHL star, "What the f**k were you thinking!?"
It's all in new police video obtained by TMZ Sports ... in which the female security guard tells an officer all about a May 26 incident where she says Matthews and his friends played a VERY unfunny joke on her.
She claims she was working a night shift at Matthews' residence in Scottsdale, Ariz., when the hockey player and 2 of his friends tried to open her locked car door at around 2 a.