“Tump” means to “spill,” “tip,” “dump,” or “knock over.” The exact origin is unknown, but “tump” is popularly used in Texas.
 
   
(Oxford English Dictionary)
tump, v.
U.S. dial. and colloq.
[Prob. repr. a colloq. pronunc. of THUMP v.; cf. TUMP-TUMP n.] 
trans. To strike (a person) forcibly; to pound, thump. Also, to knock down or over roughly.
1893 S. CRANE Maggie x. 89 ‘I’ll tump ‘im till he can’t stand.’.. ‘What’s deh use! Yeh’ll git pulled in!’ 1926 E. WALROND Tropic Death i. 28 Shut up befor’ I tump yo’ down! 1931 Amer. Speech VII. 31 Negro Vocabulary… tump v., thump. 1977 Amer. Speech 1975 L. 68 Tump over, tip or knock over. ‘Don’t tump over that glass!’ 1983 Dallas Morning News 22 May F1/5, I wuz gonna take a big drank of muh Arro Cee Cola until you came by and tumped it over.
 
Texas Slang Translation
tump
to spill, as in “I jes’ tumped over mah beer”
 
Take Our Word For It 
Tump is a great Texas word!  Thanks for bringing back more language memories!  Of course, tump could have arisen as a cross between tip and dump.  Such words are called portmanteau words.  For example brunch - a combination of breakfast and lunch.

Mike says that, in Wales, a tump is a hillock.  It is an Anglicization of the Welsh twmp.