expression
not to be underestimated; not to be trifled with
Used when someone or something should be taken seriously, often because they have hidden strength, skill, or importance. Literally 'cannot be made a fool of'.
彼の実力は馬鹿にできない。
His ability is not to be underestimated.
この問題は馬鹿にできない。
This problem is not to be taken lightly.
From 馬鹿 (fool) + に (adverbial particle) + できない (negative potential form of する). Literally 'cannot make a fool of', i.e., cannot be treated lightly.