To formulate a theory, especially about some specific subject. / To speculate.

Concerned with theories or hypotheses rather than with practical matters. / Existing only in theory, not proven in reality.

Someone who constructs theories, especially in the arts or sciences.

Used other than figuratively or idiomatically: see open, attack: to begin attacking, to commence battle. / (cricket) Of a bowler: to bowl one of the first two overs.

(simile, colloquial, Britain, derogatory) Very unattractive.

(philosophy, religion) any doctrine of religious philosophy and mysticism claiming that knowledge of God can be attained through mystical insight and spiritual ecstasy, and that direct communication with the transcendent world is possible.

someone who is expert in the theory of a particular science or art / a theorist

(intransitive, idiomatic) To reveal a secret; to disclose.

(idiomatic) After a very long time; eventually; over a long period of time; more generally.

in theory; on paper / in a theoretical manner