'frequently-observed' originates from the combination of 'frequent' and 'observe', where 'frequent' meant 'occurring often' and 'observe' meant 'to watch or notice'.

'frequent' and 'observe' were combined in modern English to form the adjective 'frequently-observed'.

Initially, 'frequent' meant 'occurring often' and 'observe' meant 'to watch or notice', and this meaning has largely remained the same in modern usage.