Which term describes someone who is lacking in friendliness?

The term "aloof" accurately describes someone who is lacking in friendliness. When a person is described as aloof, it suggests that they are emotionally distant, uninvolved, or detached from social interactions. This can manifest as unapproachable behavior or a sense of indifference towards others, which creates a barrier to building relationships or expressing warmth.

In contrast, the other terms do not convey the same meaning. "Adept" refers to someone who is highly skilled or proficient at something, suggesting competence rather than social interaction. "Advocate" is someone who supports or recommends a particular cause or policy, indicating a supportive role, while "abet" refers to the act of encouraging or assisting someone, typically in a wrongdoing, which again does not pertain to friendliness.