It's just a comparative adjective...

Posted by Alex on November 28, 2001

In Reply to: The word junior posted by Bruce Kahl on November 27, 2001

: : I'd like to know the meaning of the word 'junior' in the following sentence:

: : "He married with a girl who was twenty years his 'junior'.

: : Junior in this case is not his younger son, once he married in an advanced age, and never had had son.

: It means the girl was twenty years younger than he.
: If she is 20 years his junior and he is 50 years old then she is 30 years old.

basically most words ending in '-ior' are taken straight from the Latin equivalent - for example in Latin:

positive comparative superlative
iuvenis (young) iuvenior
senex (old) senior
bonus (good) melior

melior then gives us words like 'ameliorate' - to make better.
so anyway junior just seems to be a corruption of iuvenior. the confusion of the original point comes from the fact that these adjectives when used on their own take on substantive properties.