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'.
His ability is not to be underestimated.
This problem is not to be taken lightly.
Means 'to make fun of' or 'to look down on'. 馬鹿にできない is the negative potential form, meaning 'cannot be looked down on'.
A more formal synonym meaning 'not to be underestimated'. 馬鹿にできない is slightly more colloquial.
Standard kanji-kana mixed spelling.
Variant with 出来 in kanji; less common but still seen.
Katakana spelling of 馬鹿; adds a colloquial or emphatic tone.
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.