Roget's Thesaurus: Entry 612 (Impulse)

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#612. Impulse. -- N. impulse, sudden thought; impromptu, improvisation; inspiration, flash, spurt. improvisatore[obs]; creature of impulse.

V. flash on the mind.

say what comes uppermost; improvise, extemporize.

Adj. extemporaneous, impulsive, indeliberate[obs]; snap; improvised, improvisate[obs], improvisatory[obs]; unpremeditated, unmeditated; improvise; unprompted, unguided; natural, unguarded; spontaneous &c. (voluntary) 600; instinctive &c. 601.

Adv. extempore, extemporaneously; offhand, impromptu, a limproviste[Fr]; improviso[obs]; on the spur of the moment, on the spur of the occasion.