exact syllable matches

own case

must be unassailable

with 1 extra syllable

Back up is

currently unavailable

 

history of the ancient world

 

Possibly a substratum word

 

Probably a substratum word

 

Suddenly a truck races up

ideal that is

totally unattainable

with 2 extra syllables

Also,

apparently, unsustainable

long-term, though, I'm

emotionally unavailable

Uncertainty is

inherently unsustainable

an enormous undertaking,

light may be

practically unattainable

with 3 extra syllables

and Felicity is

momentarily unavailable