Adolescence /ˌæd

It 's highly improbable that Norris will agree.
an improbable-sounding excuse
insoluble /ɪnˈsɒl.jʊ.bl ̩/ /-ˈsɑːl.jə-/ adjective PROBLEM
1. ( US also insolvable ) (of a problem) so difficult that it is impossible to solve
Traffic congestion in large cities seems to be an insoluble problem.
insoluble /ɪnˈsɒl.jʊ.bl ̩/ /-ˈsɑːl.jə-/ adjective SUBSTANCE
2. (of a substance) impossible to dissolve
These minerals are all insoluble in water.
irreparable /ɪˈrep.rə.bl ̩/ adjective
impossible to repair or make right again
Unless the oil spill is contained, irreparable damage will be done to the coastline.
irreplaceable /ˌɪr.ɪˈpleɪ.sə.bl ̩/ adjective
too special, unusual or valuable to replace with something or someone else
Most of the porcelain you see in the display cabinets is irreplaceable.
No-one's irreplaceable in the workplace.
irreversible /ˌɪr.ɪˈvɜː.sɪ.bl ̩/ /-ˈvɝː-/ adjective
not possible to change; impossible to return to a previous condition
Smoking has caused irreversible damage to his lungs.