to save up; to stockpile; to stash away; to hoard; to amass; to salt away
長年疲れを溜め込んだ。
I built up fatigue over many years.
彼女はずっとストレスを溜め込んでいる。
She has been bottling up stress for a long time.
不満を溜め込むのは体に悪い。
Bottling up frustration is bad for your health.