English word forthink comes from English for-, English think, Old English þenċan, Old English fōre-, and later Old English foreþencan (To think beforehand; forethink.)

Detailed word origin of forthink

Dictionary entry	Language	Definition
for-	English (eng)	(dialectal) Very; excessively.. (no longer productive) Meaning "completely", "to the fullest extent" e.g. forbreak; superseded by combinations with "up" in senses where no upward movement is involved, e.g. forgive = give up (one's offenses), forgather = "gather up", forbeat = "beat up", etc.. (no longer productive) Meaning "far", "away"; "from", "out" e.g. forbid, forget, forsay; forbear, fordeem.
think	English (eng)	(intransitive) To communicate to oneself in one's mind, to try to find a solution to a problem.. (intransitive) To conceive of something or someone (usually followed by of; infrequently, by on).. (transitive) To be of the opinion (that).. (transitive) To consider, judge, regard, or look upon (something) as.. (transitive) To guess; to reckon.. (transitive) To ponder, to go over in one's [...]
þenċan	Old English (ang)
fōre-	Old English (ang)
foreþencan	Old English (ang)	To think beforehand; forethink.
forthynken	Middle English (enm)
forthink	English (eng)	(intransitive, obsolete) To repent, be sorry for.. (reflexive, obsolete) To regret; repent.. (transitive, obsolete) To change one's mind about; to renounce.. (transitive, obsolete) To regret.. (transitive, obsolete, impersonal) To cause distress or regret to; cause to regret or repent; to vex.

Words with the same origin as forthink