'non-SI' is formed from the prefix 'non-' plus the abbreviation 'SI'. The prefix 'non-' comes from Latin 'non' meaning 'not', and 'SI' is an abbreviation of the French 'Système international (d'unités)'.

The abbreviation 'SI' was established in the mid-20th century (from French 'Système international') to name the International System of Units. The construction 'non-' + 'SI' arose later as a straightforward negation to label units or practices outside that system; thus 'non-SI' developed as a compound describing such units.

Initially and subsequently it has meant 'not part of the SI system' (i.e., 'outside the International System of Units'); this core meaning has been stable over time.