'Ig' originates from English, specifically as the abbreviation of the word 'immunoglobulin' (formed from 'immuno-' + 'globulin').

'immunoglobulin' was coined in modern scientific/medical usage (20th century) from 'immune' + 'globulin'; the shortened form 'Ig' came into use in immunology literature as an abbreviation.

Initially used to refer to globulin proteins related to immunity ('immunoglobulins'); the abbreviation 'Ig' has retained that meaning in scientific contexts and additionally appears as unrelated internet slang ('I guess').