Kick it in the ass was the catch phrase of Supernatural producer and director Kim Manners, who died in January 2009.
The phrase has been used repeatedly throughout the series as a tribute to Manners. The phrase first showed up in Season 5 onwards.
In Swan Song, Dean Winchester loads what is commonly referred to as "Dean's last mix tape" into the cassette deck while on his way to confront Lucifer at Stull Cemetery. The mix tape is labeled "Kick It In The Ass".
In The Girl With The Dungeons and Dragons Tattoo, after Sam gives Charlie the "What would Hermione do?" pep speech, Charlie answered with "She kicked ass" and promise to "Kick it in the ass" herself. According to Charlie's actress, Felicia Day, the line was originally "Kick Ass" but changed on the suggestion of director John MacCarthy.