'suddenly-grown' is a compound of 'suddenly' and 'grown'. 'suddenly' originates from Middle English and Old French 'soudain', ultimately from Latin 'subitaneus' (from 'subitus'), where 'subitus' meant 'sudden' or 'unexpected'. 'grown' is the past participle of 'grow', which comes from Old English 'growan' meaning 'to grow'.

'suddenly' developed in Middle English from Old French 'soudain' (itself from Latin 'subitaneus'), while 'grown' evolved as the past participle form of Old English 'growan'; the two words have been combined in modern English as a hyphenated compound (e.g., 'suddenly-grown') to describe rapid growth.

Individually, 'sudden'/'suddenly' originally meant something occurring quickly or unexpectedly and 'grow' meant to increase in size; combined as 'suddenly-grown' the phrase specifically emphasizes rapid or abrupt growth—this usage is a modern compounding rather than a single inherited lexical item.