I'm not positive, but I think giving "a boot in the ass" is another way of saying that something or someone needs "a kick in the pants."
"a kick in the butt" also "a kick in the pants":
forceful encouragement to do something
To increase in severity, you could add stiff, as in: "A stiff kick in the pants."
EDIT: This increase in severity stands out to me. My argument is that "a boot in the ass" is just a harsher way of saying "a kick in the pants" and, in the opposite direction, I think the phrase "rip a new asshole," as in "after your performance on the field yesterday, the coach is going to rip you a new asshole," is a harsher iteration still. An ngram search on all three terms: "kick in the pants", "boot in the ass", and "rip you a new asshole", finds that "a kick in the pants" became increasingly popular from 1925-1940 and then dropped off dramatically around 1944. This is about the same time that "a boot in the ass" registers in Google's records. It's not until 1979 that "rip you a new asshole" registers. "Kick in the pants" is still the most commonly used expression by a large amount--the other two are not "fit to print" by most standards--but it looks like we are finding harsher and harsher iterations.
Out of curiosity, I did the same search for the following three phrases that, arguably, increase in magnitude as well:
Give a damn.
Give a shit.
Give a fuck.
A similar pattern occurs.
Google registers hits for "don't give a damn" as far back as 1867; "give a shit" doesn't show up with any regularity until 1929; and "give a fuck" registers 12 years later in 1941. What's also interesting about this particular grouping is the sharp drop off "don't give a damn" follows, and, to a lesser extent, "don't give a shit", starting in 2002. "Don't give a fuck" is the only phrase seeing its numbers increase.
Maybe this is intuitive, maybe not. What I take away is that English speakers like to hone the edge of a dulled expression.