misunderestimate

“Misunderestimate” is a malapropism invented by President George W. Bush that has come to mean “to underestimate by mistake.”

Bush accidentally used the term in a 2000 interview, saying, “They misunderestimated me.” He likely meant to say “underestimated” but the term became linked with Bush, often as evidence for his lack of intelligence.

Source: Taegan Goddard’s Political Dictionary