[franksolich]

Well, if I remember Latin correctly, it means "beware the fury of a patient man."

But perhaps the Latin is rusty?

[Duke Nukum]

I got a similar sentiment when I googled the phrase. From Bartlett's:

NUMBER: 2939
AUTHOR: John Dryden (1631-1700)
QUOTATION: Beware the fury of a patient man. Note 1
ATTRIBUTION: Absalom and Achitophel. Part i. Line 1005.

Note 1. Furor fit læsa sæpius patientia (An over-taxed patience gives way to fierce anger.—Publius Syrus: Maxim 289.