From platitude + -in- + -ous, probably after multitudinous.
platitudinous (comparative more platitudinous, superlative most platitudinous)
- Characterised by cliches or platitudes.
- 2005, Christopher Hitchens, "André Malraux: One Man's Fate", New York Times Book Review, 10 Apr 2005:
- The meeting was brief and platitudinous, but in later accounts [...] Malraux turned it into a major summit of great minds.