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back; rear
background; backdrop
backing; support; backer; supporter
going backwards; reversing; backing up
reverse (gear)
refunding; returning (e.g. profits)
back (player)
backstroke (swimming)
backhand
doggy style (sexual position)
バックお願いします。
Please back up.
バックミラーを見る。
Look in the rearview mirror.
バックドアを開けて荷物を積んだ。
I opened the back door and loaded luggage.