godan verb (-ku)
lose interest; slacken off
Rare standalone verb form of 気を抜く; used to mean losing focus or relaxing one's attention. Almost always encountered as the phrase 気を抜く in modern Japanese.
See also: 気を抜く (きをぬく)
The form 気抜く can be found in older texts, but in modern Japanese 気を抜く is the usual expression.