Roget's Thesaurus: Entry 137 (Infrequency)

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#137. Infrequency.-- N. infrequency, rareness, rarity; fewness &c. 103; seldomness[obs]; uncommonness.

V. be rare &c. adj.

Adj. unfrequent[obs], infrequent; rare, rare as a blue diamond; few &c. 103; scarce; almost unheard of, unprecedented, which has not occurred within the memory of the oldest inhabitant, not within one's previous experience; not since Adam[obs].

scarce as hen's teeth; one in a million; few and far between.

Adv. seldom, rarely, scarcely, hardly; not often, not much, infrequently, unfrequently[obs], unoften[obs]; scarcely, scarcely ever, hardly ever; once in a blue moon.

once; once in a blue moon; once in a million years; once for all, once in a way; pro hac vice[Lat].

Phr. ein mal kein mal[German].