Smashing

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early 18th century (as a noun): probably imitative, representing a blend of words such as smack, smite with bash, mash, etc.

wiktionary

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From smash +‎ -ing. As a synonym for wonderful, the term first appeared in early 20th-century USA, and possibly derives from the sense of smash used in smash hit and similar terms. Popular folk etymology connects the term to the Irish is maith sin or Scottish Gaelic 's math sin ("that is good"), but this has been described as "improbable", [1] and does not appear in most dictionaries.

etymonline

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smashing (adj.)

1833, "violently crushing to pieces," present-participle adjective from smash (v.). Meaning "pleasing, sensational" is from 1911. Related: Smashingly.