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to amass; to accumulate; to store
to leave unpaid
旅行のためにお金を溜めている。
I'm saving money for a trip.
家賃を三ヶ月も溜めてしまった。
Ended up leaving three months of rent unpaid.
彼は不満を溜め込んでいて心配だ。
He had been accumulating frustrations.