Green’s Dictionary of Slang

understumble v.

to understand.

1577		Misogonus in Farmer (1906) I iv: You understumble me well, sir, you must have good wit.
c.1681		E. Hickeringill Trimmers Friendly Debate in Works (1709) I 386: Oh! ho! I begin to understumble you, Edad, I will not tell you.
1738		Swift Polite Conversation 48: miss.: I understumble you, Gentlemen. nev.: Madam, your humblecumdumble.
1895		Crowe Aus. Sl. Dict. 90: Understumble, to understand.