'occasionally-observed' is a compound word formed from 'occasionally' and 'observed'. 'Occasionally' originates from the Latin word 'occasio', meaning 'a happening' or 'an opportunity', while 'observed' comes from the Latin 'observare', meaning 'to watch over'.

The term 'occasionally-observed' combines the adverb 'occasionally' with the past participle 'observed' to describe something seen at irregular intervals.

Initially, 'occasionally' meant 'at times' and 'observed' meant 'watched over'. Together, they describe something seen at irregular intervals.