To ruin or botch, to blunder. Undoubtedly alluding to the unhappy condition of being “loused up,” that is, infested with lice, this slangy term dates from the first half of the 1900s. At first it was used as a transitive verb, as in John O’Hara’s Appointment in Samarra, “Lousing up your date.” A decade or two later it was also being used intransitively, as in “Don’t trust her with the reservations; she’s sure to louse up.”