Meaning
If you can't catch Quaco himself, catch his shirt. When you cannot get at the person or thing you really want, take hold of whatever part of it you can — get what is within reach rather than nothing at all. Often used of taking revenge or recompense on whatever is nearest to the one who wronged you.
Examples
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Di man gaan, so dem tek it out pon him bredda — if yuh cyaa ketch Quaco, yuh ketch him shut.
The man's gone, so they took it out on his brother — if you can't get the one you want, get what's nearest.
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Get what yuh can from di deal; if yuh cyaa ketch Quaco, yuh ketch him shut.
Get what you can from the deal; take hold of whatever's within reach.
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Dem couldn't reach di boss, so di worker ketch it; if yuh cyaa ketch Quaco, yuh ketch him shut.
They couldn't reach the boss, so the worker caught it; you seize whatever part you can.
Origin
'Quaco' is an Akan day-name (a boy born on Wednesday), common in old Jamaica — here standing for any elusive person. If the man himself slips away, you at least grab his shirt.
How it’s used
Used of settling for a partial gain, or of taking recompense/revenge on whatever is nearest when the real target is out of reach.