بالصيف ضيعت اللبن
You wasted the milk in summer.
The well-known proverb is used of someone who let a useful opportunity pass at the appropriate time and later seeks what is no longer available. Its rhetorical force lies in summoning a remembered complete incident through a compact phrase.
In later applications, “milk” need not refer to actual milk, yet the point is not merely substituting a different word for the noun. The whole saying represents a recurring situation: voluntarily losing an opportunity and regretting it after the fact.