In manga and anime, a large sweat drop, shaped like teardrop, dripping from a character's forehead or hair, is a symbol used when a character is perplexed, bewildered, confused, or otherwise at loss of words.
It's also known as the "anime sweat drop." In Japanese, it's called ase-maaku 汗マーク, "sweat mark (mark as in a symbol, icon)."
See small sweat drop for a similar but smaller symbol.
Anime: Hayate no Gotoku! ハヤテのごとく！ (Episode 11)