Middle English (in an earlier sense). From rife + -ly. Compare Middle Dutch rivelike, rivelijk, rivelic, riveleec abundantly, amply, Old Icelandic ríflega generously, Old Swedish riflika copiously, abundantly, Old Danish rivelige, rivlig abundantly.
Definition of rifely in US English:
rifely
adverbˈrʌɪfli
Commonly, frequently; customarily.
Origin
Middle English (in an earlier sense). From rife + -ly. Compare Middle Dutch rivelike, rivelijk, rivelic, riveleec abundantly, amply, Old Icelandic ríflega generously, Old Swedish riflika copiously, abundantly, Old Danish rivelige, rivlig abundantly.