1.Definition: not disposed to friendship or friendliness

"An unfriendly coldness of manner", "An unfriendly action to take"

2.Definition: not easy to understand or use

"User-unfriendly"

3.Definition: not friendly

"An unfriendly act of aggression"

Related Adjective(s):inimical

4.Definition: very unfavorable to life or growth

"The unfriendly environment at high altitudes"

Related Adjective(s):hostile, uncongenial