Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class I. Words Expressing Abstract Relations
Section VI. Time
3. Recurrent Time

137. Infrequency.

   NOUN:INFREQUENCY, infrequence, unfrequency [rare], rareness, rarity; sparseness, fewness [See Fewness]; seldomness; uncommonness.
   VERB:BE RARE &c. adj.
   ADJECTIVE:INFREQUENT, unfrequent [rare], seldseen [archaic], uncommon, sporadic; rare, rare as a blue diamond; few [See Fewness]; scarce; almost unheard of, scarce as hen’s teeth [colloq.], unprecedented, which has not occurred within the memory of the oldest inhabitant, not within one’s previous experience.
   ADVERB:SELDOM, rarely, scarcely, hardly; not often, unfrequently, infrequently, uncommonly, sparsely, unoften; scarcely ever, hardly ever; once in a blue moon [colloq.].
  once; once for all, once in a way; pro hac vice [L.].
  1. Ein mal kein mal.
  2. Like angel visits, few and far between.—Campbell