back-to-back
back-to-back {adv.}
1. Immediately following.
The health clinic had back-to-back appointments for the new students during the first week of school.
2. Very close to, as if touching.
Sardines are always packed in the can back-to-back.
The bus was so full that people had to stand back-to-back.
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Source: A Dictionary of American Idioms
