Actually, that common turn of phrase is itself rather daft. It sounds like I took some time off in order to be unwell.
Be that as it may, I was wondering about a small group of words whose meanings and derivations overlap, but which are not all that close, after all. Usually, on a post like this, I would start with the research into derivation and past usage, but I want to be clear first on what I think are the usual modern meanings of this small group of words, to whit:
Recover - the most commonly used, with the widest variety of meanings, all of which orbit around notions of getting back something lost, to which you have title or ownership or otherwise deserve. In my case, recovery from ill health is the recovery of good health. But you can also recover your investment, your stolen goods and your long lost sister.
Regain - seems to be more specific and more abstract than recover at the same time, since you can regain both the shore having been blown adrift, and also regain consciousness having been knocked out. Soldiers can regain ground that was lost, and also regain their former glory after being dishonoured. Some of these things can also be recovered, but unlike recover, regain has not yet made the leap to intransitivity. You always have to regain something.
Recuperate - almost exclusively used in the expression R&R - rest and recuperation. It seems to mean something very similar to recovery - to restore a depleted state - but suggests a more active, concerted and/or directed approach.Whereas I have been languishing in my bed, passively awaiting recovery, soldiers on leave seem to have a whole host of things they do in order to recuperate their physical and mental good condition. Some of those things seem quite strenuous and even risky.
Recoup - used almost exclusively in the expression recoup your losses, it seems to imply in most modern usage some sort of damage limitation, often after a risky investment.
NOW I'm going to visit my lexicography shelf. Back in a mo.
Recover - I have not been surprised to discover that this is etymologically the same word as recuperate - with the latter coming direct from Latin and recover coming via medieval Latin and French, which explains the small vowel shift and transformation of 'p' to 'v' (transformations between p, b, v, l and r are commonplace). Has a acquired a wide range of uses across the ESW but in all cases about getting back something lost - generally quite recently, and often figurative or abstract.
Regain - same root as again - which is 'gain' - being profit, or more specifically (in ancient usage) yield of crops. Regain is complicated, though. The modern (if ancient) French word regain refers to hay collected a second or third time from the same field - the sense of yield being very strong, but illustrates well the sense of profiting more than once from the same endeavour, which was the sense of regain for a long time. However in English the sense of getting back something lost has almost completely eclipsed this sense. We seem to prefer regain over recover in certain situations, and many writers prefer if for the spurious virtue of being "less Latin".
Recuperate - Etymologically, this means to recapture. Usage seems to be largely interchangeable with "recover" though the Latin haters hate this even more. Most usage centres around physical condition or loss of assets or investments. A telling distinction is that you can recover a lost cargo, but you can't recuperate a lost cargo. However freight can be recuperated. Heh...
Recoup - by far the most interesting of the bunch, the "coup" is to cut. It was a legal term, meaning to recover or redress a loss, such as by deducting expenses. As such the most common usage is to indemnify or otherwise make up losses, typically in investments.