Like most of the 'th~' words, you can expect a Viking influence on the language. OED also posits a possible relationship with dowel.

OE. þol(l, corresp. to ON. þollr, Norw. toll, tulle, Sw. (år) tull, Da. (aar) tol; MLG. dolle, dulle, dole, doule, LG. (Brem. Wbch.) dolle, dulle, EFris. dolle, dol, MDu. dolle, Du. dol(l. Ulterior etymology uncertain. In ON. þollr was also ‘fir-tree’, poet. ‘tree’ generally: the connexion of sense is not clear. The history of the Eng. word also shows a hiatus during nearly the whole ME. period.
The late altered forms thoule, thowle, and 19th c. thowel, may be influenced by doule, dowle, dowel.]

1. A vertical pin or peg in the side of a boat against which in rowing the oar presses as the fulcrum of its action; esp. one of a pair between which the oar works; hence, a rowlock.
c725 Corpus Gloss. (O.E.T.) 1820 Scalmus, thol. c1000 Voc. in Wr.-Wülcker 289/9 Scalmus, ðoll. 1611 Cotgr., Scalme, a Thowle; the little peg whereby the oare of a Skiffe is staied. 1624 Capt. Smith Virginia 62 In stead of thoules wee made stickes like Bedstaues. 1697 W. Dampier Voy. round World (1699) 35 Straps+through which they put their Oars in rowing, instead of tholes or pegs. 1769 Falconer Dict. Marine (1789), Autarelles, the thoules or rowlock-pins of a galley. 1827 Roberts Voy. Centr. Amer. 178 These oars are secured to the thowel by straps of raw hide. 1847 Longfellow Evang. ii. ii. 102 The sound of their oars on the tholes had died in the distance. 1857 P. Colquhoun Comp. Oarsman's Guide 29 The row~lock is composed of 3 parts; the thauel, against which you row [etc.]. 1862 Whittier Cry Lost Soul iv, The guide+drops his oar against the gunwale's thole.

2. A pin or peg in general: spec. a. A pin by means of which the shafts are fastened to the carriage or axle of a cart, etc. b. The handle or ‘nib’ of a scythe-snathe.