From the Greek for "together" & "name", it means having the same meaning
[DOC] [TLE] Synonym | Define Synonym at Dictionary.com [PAR] Synonym definition, a word having the same or nearly the same meaning as 
another in ... a word or expression accepted as another name for something, as 
Arcadia for ... 1400-50; < Latin synnymum < Greek synnymon, noun use of 
neuter of .... of synonymos "having the same name as, synonymous," from syn- "
together,... [DOC] [TLE] All the same | Define All the same at Dictionary.com [PAR] All the same definition, identical with what is about to be or has just been ... being 
one or identical though having different names, aspects, etc.