1. In a manner disapproved or not allowed by custom
   "He acted illegitimately when he increased the rent fourfold"
   - illicitly 
2. Of biological parents not married to each other
   "this child was born illegitimately"
   - out of wedlock

See also: illegitimate

Antonym: legitimately

• factious
• factiously
• factitious
• factitive
• factoid
• factor
• factor analyse
• factor analysis
• factor analytic
• factor analytical
• factor analyze
• factor I
• factor II
• factor III
• factor in
• factor IV
• factor IX
• factor of proportionality
• factor of safety
• factor out
• factor V
• factor VII
• factor VIII
• factor X
• factor XI