Done or said without previous thought or preparation; casual, impromptu, or spontaneous. As an adverb, it means immediately or without checking references.
Originally meant 'without using the hands for support' in archery and shooting (off the hand rather than from a rest), dating from the 16th century. The figurative sense of 'casual, unpremeditated' developed by the 17th century, suggesting something done easily without special preparation.
The archery origin of 'offhand' reflects a fascinating shift from physical to mental metaphor - just as shooting without hand support requires skill and confidence, speaking 'offhand' suggests someone comfortable enough with a subject to discuss it without preparation or notes.
Complete word intelligence in one call. Free tier — 50 lookups/day.