Meaning:
1. mouth open wide, gape, agape
2. open mouth in wonder/amazement

Usage Example:
1. 弟は美味しそうなハンバーガーが運ばれてくると、あんぐりと口を開けて食べ始めた。 (After getting a tasty looking hamburger, my brother opened his mouth wide and started eating.)
2. 彼の合計点数を聞き、姉はあんぐりとしたまま何も言えなくなっていた。 (After seeing his total score, his sister just stood there speechless and with an open mouth.)

Variations:
あんごり