孤假虎威的英文?

It should be "狐假虎威" but not "孤假虎威",

More close to the literal meanings:
"The fox borrows the tiger's power"
"The fox fakes while the tiger threatens"

More close to the underlying meanings:
"bully people by flaunting one's powerful connections"

Orphaned false awe-inspiring prowess

Orphaned false awe-inspiring prowess

孤假虎威
Assume someone else's authority to browbeat others

Formal:
John ingratiated himself with the new president in the party so other guests will pay attention to him.

Not so formal:
John sucks up to the big guy 'cause other guys will kiss his ass, he thinks.