'non-routable' originates from the negative prefix 'non' (from Latin 'non', meaning 'not') combined with 'routable', which is formed from the noun 'route' (from Old French 'route', meaning 'way, road') plus the adjective-forming suffix '-able' (from Latin '-abilis').

'route' entered English from Old French 'route' (meaning 'way, path'), 'routable' was formed in English by adding the suffix '-able' to indicate 'capable of being routed', and the modern compound 'non-routable' was created by prefixing 'non-' to negate that meaning.

Initially the elements meant 'not' (for 'non-') and 'capable of being routed' (for 'routable'); over time the compound 'non-routable' came to be used specifically in technical networking contexts to mean 'not reachable or forwardable by routing'.