Learn English – Origin of “happy camper”

Wikipedia has an article on third degree interrogation. Here are its hypotheses as to its origin:

No one knows the origin of the term but there are several hypotheses. [...]

Possible origins

• The third degree of Freemasonry and the rigorous procedures to advance to that level.
• The term may have been coined by Richard H. Sylvester, the Chief of Police for Washington, DC. He divided police procedures into the arrest as the first degree, transportation to jail as the second degree, and interrogation as the third degree.
• The term may have been coined by nineteenth century New York City Police detective Thomas F. Byrnes, perhaps as a pun on his name, as in third degree burns.

As an aside, you may be interested in this popular and related question.

Phrases.org seems more definite about the origin:

In Masonic lodges there are three degrees of membership; the first is called Entered Apprentice, the second Fellowcraft, and the third is master mason. When a candidate receives the third degree in a Masonic lodge, he is subjected to some activities that involve an interrogation and it is more physically challenging than the first two degrees. It is this interrogation that was the source of the name of the US police force's interrogation technique. That is referred to in an 1900 edition of Everybody's Magazine:

"From time to time a prisoner... claims to have had the Third Degree administered to him."

Answers.com and Dictionary.com seem to agree:

third degree
"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony.

Grill meaning to subject to severe and persistent cross-examination is a metaphor. Imagine the examinee on a grill, being intensely "cooked" through by the questioning. Etymonline says its first usage in this sense is attested in 1894.

Meaning

Etymonline.com says of handy:

Meaning "conveniently accessible" is from 1640s.

To come in handy means something will be handy or useful in the near future, that it will become handy.

Come in handy

The earliest verifiable examples I found in Google Books are both from 1843.

First, in a "Weekly Journal of Gardening" column of The Gardener and Practical Florist:

CELERY, as we always recommend in small gardens, should be planted out at various seasons, and if there be any left in the seed bed, another row will come in handy. Earth up that which is advancing. LETTUCES in the seed bed may be thinned, and those taken out may be planted.
July 15, 1843.

Second, in Tales of the Colonies, or, The Adventures of an Emigrant, Volume 2:

"What have we got here ? a pair of handcuffs ; ah ! these come in handy ; the bushranger won't want handcuffs any more, but they'll do for his mate."

Come handy

We can also find some slightly early uses of the similar to come handy. It was once used similarly and as often to come in handy, but lately has become rarer.

An October 1824 The London Magazine prints a letter from summer 1821:

please your oner,
hoping your oner wont be displeasd at my boldness and I send a little basket of eggs-good fresh eggs-and they were lade by the little black hen that's three yeer ould come Michaelmas eve the day that I send home your oner's shute— and the times are very hard intirely — intirely — plase your oner from
your oner's sarvent to comand,
Timotheus Kinnealy.
the woman hopes the eggs wil come handy to the young mistris out of her confinement. — tuseday mornin.