Connected with; pertaining to.
[Originally from Latin adjectives ending in -arius.]
In the same way that many Latin words ending in -us arrived in English through French with the ending changed to -ous, some of those in -arius changed to -arious (others took on the endings -arian or -ary2 instead).
Words in -arious are adjectives. Some common examples are gregarious (Latin greg-, grex, a flock or herd), fond of company or sociable; precarious (Latin prex, prec-, prayer), uncertain or insecure; nefarious (Latin nefas, nefar-, wrong), wicked or criminal; vicarious (Latin vicarius, substitute), of something experienced through the feelings or actions of another person; and various (Latin varius, changing or diverse).
This suffix has never been active in English; examples that appear to exist have actually been formed by attaching -ous (or its variant -ious) to a word stem containing -ar-: burglarious, hilarious, uproarious.