Entry: insousistent (adj.)
In context: "...Lenz was able to play the whole thing off with insousistent aplomb, sitting there in his aviator sunglasses with his legs crossed and his topcoated arms resting along the backs of the empty chairs on either side."
Definition: Definitely a Lenzism. Probably he's thinking of something along the lines of insouciant (adj.): Careless, indifferent, unconcerned.
Previously featured on Definitive Jest.
Other: I like this usage (again!):
1888 Pall Mall Gaz. 12 Jan. 2/1 On such subjects, an insouciant agnosticism is the most philosophic attitude.
SNOOT score: 1
Source: Oxford English Dictionary