Reverse Dictionary: THINK

ADJECTIVES
1618 ► thoughtful, pensive; absorbed in thought → MUSEFUL
1662 ► consisting in mere thinking; speculative → THINKATIVE → obs.
1702 ► not having the faculty of thinking → INCOGITANT 
1999 ► in a daydream; deep in thought → → IN A BROWN STUDY (Bk.)

NOUNS
1330 ► the process of viewing or considering mentally; thought, thinking, consideration → ADVISEMENT → obs.
1340 ► the act of thinking, considering, reflecting, or remembering → BETHINKING 
1589 ► that which, tires the brain by constant thinking → TIRE-BRAIN → obs.
1853 ► an impediment to thinking clearly; inability to concentrate, remember, etc. → BRAIN FOG 
1855 ► the product of thinking; ideas → BRAIN-STUFF 
1907 ► a moment when one is unable to think clearly or act sensibly → BRAINSTORM → sl.
1980 ► confused, muddled thinking → BACKWARD THINKING → African-American sl.
1990 ► something that makes one think → ASS-SCRATCHER → US sl.
1991 ► a mania for thinking → PHRONEMOMANIA → (Bk.)
1991 ► an abnormal fear of thinking → PHRONEMOPHOBIA → (Bk.)
2006 ► a lapse in thinking → BLOND MOMENT → US sl. (Bk.)
2006 ► fast and clever thinking → FANCY FOOTWORK → US sl. (Bk.)
2006 ► fast and clever thinking → FAST FOOTWORK → US sl. (Bk.)

NOUNS, PERSON
1589 ► one who, or that which, tires the brain by constant thinking → TIRE-BRAIN → obs.
1602 ► one who thinks and does brain-work for another → BRAIN-WRIGHT 
1626 ► one who holds an opinion; a thinker; a theorist → OPINATOR → obs.
1832 ► a thinker who gives precedence to abstract ideas and concepts over concrete and particular ones → ABSTRACTIONIST 
1914 ► an original thinker → SANTA CLAUS → criminals’ sl. (Bk.)
1945 ► a fast-moving person; a rapid runner; a quick thinker → SWIFTIE → SWIFTY

PHRASES
L19 ► use your (head) brains → USE YOUR LOAF → UK sl.

VERBS
1000 ► to think of or about; to bear in mind → BETHINK → obs.
1000 ► to think, to suppose → I-WENE → obs.
1175 ► to think of, to imagine, to conceive → BETHINK 
1200 ► to think → LET → obs.
1200 ► to think about, to consider; to remember → UMBETHINK → obs. exc. Sc. obs
1200 ► to think about, to consider; to remember → UNBETHINK → obs. exc. Sc. obs
1220 ► ​to think over a thing with a view to decision or action; to consider → BETHINK → obs.
1275 ► to think, to suppose, to believe something to be the case → AWEEN → obs.
1300 ► to think, to consider, to reflect, to meditate → BETHINK → arch.
1430 ► to think of a thing, to consider → ADVERT → obs.
1529 ► to be able to think; to remember → BEAR A BRAIN → obs.
1532 ► to spend time trying to think about something, esp. something considered trivial → BUSY ONE’S BRAINS 
1555 ► to think, to suppose, to opine → OPINION → obs.
1579 ► to think persistently and laboriously → BEAT THE HEAD 
1593 ► to think persistently and laboriously → BEAT THE BRAINS 
1603 ► to think long and hard → PUZZLE ONE’S BRAIN(S)→ rare
1604 ► to make a tremendous effort to think about or remember something → CUDGEL ONE’S BRAINS 
1621 ► to think, to believe, to suppose → OPINIONATE → rare
1624 ► to think, to suppose, to opine → OPINIATE → obs.
1655 ► to make a great effort to think about something → CRACK ONE’S BRAIN(S) 
1737 ► to think hard; to beat one’s brains → YARK → YERK → Sc.
1791 ► to think, to reckon, to deem → RECK → Sc.
19C   ► to think, to muse  → DERN → Sc.
1830 ► to think, to expect, to believe → CALCULATE → US colloq.
1863 ► to conjecture, to think → BEAR IN HAND → Eng. dial.
1877 ► to think; to having an inkling or suspicion of → MISDOUBT → Sc. & Eng. dial.  
1888 ► to think, to consider, to estimate → CALL → Eng. dial.
1895 ► to make a mental effort; to think deeply over → SET THE BRAINS ASTEEP → Sc.
1896 ► to think, to believe → FANCY → Eng. dial.
20C   ► to think for oneself → USE ONE’S NOODLE → Aust. sl.
1901 ► to think; to consider → TAKE → Sc. & N. Eng. dial.
1919 ► to think out; to solve a problem, plan of action, etc. → NUT OUT → Aust. sl.
1942 ► to think, to think over, to study → NOODLE → US sl.
1947 ► to think; to use the mind → TRIG THE WIG → US sl. (Bk.)
1970 ► to think, to reckon → SCHEME → S. Afr. sl.
1999 ► to think about → CHEW OVER (Bk.)