Many of Bellairs's made-up words appear in his books, most notably St. Fidgeta & Other Parodies.
Bowen explains that some of these spontaneous notes were a sort of one-man brainstorming session (brainstorming burst on the scene in the mid-50s) and "are the verbal equivalent of doodling. But since little pictures wouldn't work, John throws in a lot of meaningless verbalization to create the effect of someone waiting 'with his mind in neutral' (as the expression is) for the next bright idea to arrive out of the ether." Myers adds that, for words like ogmus, John didn't need any outside sources in the creation process: "he loved to make silly names just for the sound of them."
Other words, like tweb, Bowen remembers that Phil Gibson (of the Notre Dame College Bowl team), "whose humor had a weird streak, invented several silly words, one of which was 'tweb.' I don't recall that he invented any meanings for them, just the words, but I'm sure that John was remembering this one."
Word List
Clapp
Crapple • see Crapple, Anthony Adverse
Dropsus • see Comnenus, Isaac
Frem
Harg • see Gorboduc
Humpf
Ishbar • see Alhambra
Ogmus
Plotch • see Sunday Intruder
Rugg
Samp
Tickety-boo
Tiddy-pom
Tumpty-um-tum
Tweb
Ulfag • see Gorboduc
Wisney
Znerb