He kept exercising for three months, but recently started lapsing into laziness again.

lapsing 是动词 lapse 的现在分词形式。lapse 本意是"失误、过失"，引申为"退步、回到原来的不良状态"。 这里 lapsing into laziness 表示"又回到懒惰的状态"，即"又开始偷懒"。