My memory of the word goes back to the usage of "skew-whiff" - meaning not straight; whether for the hanging of a picture or the route taken to get somehwere. From that, it was applied to drunkeness (fairly obvious connection) and somewhere along the line became condensed to "squiffy." By the end of the War (WWII), it use was common, not just among the upper classes, where it almost certainly originated. It would be picked up (along with all sorts of other things!!!) by American Servicemen in the contacts with us Brits, and taken back "Stateside."