自不量力

to overestimate one's capabilities (idiom)

self / oneself / from / since / naturally / surely

no; not so / (bound form) not; un-

to measure

capacity / quantity / amount / to estimate / abbr. for 量詞|量词[liang4 ci2], classifier (in Chinese grammar) / measure word

to estimate one's strength

power / force / strength / ability / strenuously