1 to be connected

2 traffic

3 transportation

4 communications

5 liaison

1 common

2 ordinary

3 general

4 average

1 to join

2 to connect

3 to link up

4 to communicate

1 private

2 personal

3 interpersonal

4 sb with whom one has a close personal relationship

5 a member of one's clique

1 private correspondence

2 personal letter

3 (computing) personal message (PM)

4 to message sb

1 lit. doesn't (even) enter a single aperture (of one's head)

2 I don't understand a word (idiom)

3 it's all Greek to me

1 grasp this fundamental point and all the rest will follow (idiom)

1 to act in collusion (idiom)

1 mutual exchange of assistance (idiom)

2 to benefit from each other's strengths and make up each other's shortfalls

3 to reciprocate with material assistance

4 scratch each other's back

1 to use one's position for personal gain (idiom)