Taking someone lightly with

(あま)() means “to underestimate” or “to take lightly”. The speaker warns that someone has failed to recognize a person’s ability, a problem’s difficulty, or a situation’s danger.

Further Reading

Examples

Don't underestimate your mother with 甘く見る

Learn the meaning and usage of the idiom 甘く見る in Japanese, with examples from manga.
怪盗セイント・テール » Volume 7 » Page 46

Meimi’s mother stops by Meimi’s room with a couple of movie tickets. She says she and Meimi’s father have already seen the movie, so she’s offering them to Meimi. She says the movie is very romantic, and suggests Meimi has someone she’d want to see it with.

芽美(めいみ):
「マ… ママ()って…」
“Mom, you knew...?”
ママ:
「ママを(あま)()ないことね」
Don't underestimate Mom.”