In sumo, failing to prevent an opponent from gaining an underarm grip; having weak defense at the armpit area.
今日は脇が甘くて、相手に下手を取られてしまった。
My underarm defense was weak today and my opponent got an underarm grip on me.
若手力士は脇が甘い試合が多いと指導される。
Young rikishi are often coached for having weak underarm defense in matches.
Being careless or off one's guard; having weak defenses or being vulnerable to attack or mistakes (idiomatic).
脇が甘いと大事な仕事でミスをするよ。気をつけて。
If you're careless you'll make mistakes on important tasks. Be careful.
彼は脇が甘くて、情報管理がずさんだ。
He's lax and sloppy with information management.
詐欺は脇が甘い人を狙うので、個人情報は厳重に管理しよう。
Scammers target people who are careless, so keep your personal information tightly controlled.