unbeknown; unbeknownst. George P. Krapp suggested that both forms are humorous, colloquial, and dialectal (A Comprehensive Guide to Good English 602 ). The Concise Oxford Dictionary of Current English likewise suggests that both are colloquial. Eric Partridge and John Simon have written, in conformity with the Oxford English Dictionary, that “unbeknown” is preferred over the dialectal “unbeknownst.” These inconsistent pronouncements serve as confusing guides. We can perhaps accept as British orthodoxy the COD’s suggestion that in British English the forms are colloquial (for “unknown”). In American English, neither can really be called dialectal or colloquial, since the words are essentially literary. In current American usage, “unbeknownst” far outranges “unbeknown” in frequency, and it must therefore be considered at least acceptable. But “unbeknownst,” like other “-st” forms (e.g., “whilst,” “amidst”), seems to come less naturally to American English. So there’s much to be said for preferring “unbeknown” — e.g.: o “Unbeknown to her, though, Christmas was the day a curse transformed him from a handsome but vain young prince into the ugly, angry Beast.” Susan King, “The Untold Chapter,” L.A. Times, 13 Nov. 1997, at F41. o “Unbeknown to the landlord, the group installed chemistry equipment, sinks, and a fume hood.” Aaron Zitner, “What Ever Happened to the Saga of RU-486?” Boston Globe, 23 Nov. 1997, at 18. Language-Change Index — “unbeknownst” for “unbeknown”: Stage 4. For information about the Language-Change Index click here. ——————– Quotation of the Day: “It is easy to affect a pompous style, to use a word twice as big as the thing you want to express: It is not so easy to pitch upon the very word that exactly fits it.” William Hazlitt, Table Talk (1821-1822), in Classics in Composition 123, 124 (Donald E. Hayden ed., 1969).