An Etymological Dictionary of Astronomy and Astrophysics

1. Older or elder (designating the older of two men bearing the same name, as a father whose son is named after him, often written as Sr. or sr. Compare → junior.
   

2a) Of higher or the highest rank or standing.

2b) (in American schools, colleges, and universities) Of or relating to students in their final year or to their class (Dictionary.com).

Etymology (EN): M.E., from L. senior “older,” comparative of senex (genitive senis) “old,” from PIE root *sen- “old;”
cf. Av. hana- “old;” O.Pers. hanata- “old age, lapse of time;” Skt. sanah “old;” Armenian hin “old;” Gk. enos “old, of last year;” Lith. senas “old,” senis “an old man;” Goth. sineigs “old,” O.Irish sen, O.Welsh hen “old.” sjunio

Etymology (PE): Mehtar “greater, elder, governor,” from meh “great, large, principla,” cognate with L. mas, → Big Bang, + comparative suffix -tar.