In Star Trek V: The Final Frontier, Kirk, Spock, and McCoy enjoy some quality time together around the campfire, musing about life, death, baked beans, and rowing boats gently down streams.

In particular, they have the following exchange:

KIRK: What are you doing?

SPOCK: I am preparing to toast a marshmelon.

McCOY: Well, I'll be damned. A marshmelon. Where did you learn to do that?

SPOCK: Before leaving the ship I consulted the computer library to familiarize myself with the customs associated with 'camping out.'

McCOY: Tell me, Spock. What do we do after we toast the marsh — er, melons?

SPOCK: We consume them.

McCOY: I know we consume them. I mean after that...

Unless I am wrong, "marshmelon" does not seem to be an alternate name for "marshmallow", at least not traditionally.

In contemporary times, the word does appear to exist, for instance in the form "Marshmallon" in Yu-Gi-Oh!, but these instances are strictly post-Star Trek V.

In the exchange, McCoy is about to say the correct term, "marshmallow", but stops himself and says "marshmelon", emulating Spock. I am tempted to think that Spock made a genuine error — one that McCoy found funny. However, Spock is not known to make errors and always chooses his words carefully. Indeed, in the preceding film we are reminded of Spock's mental prowess and precision:

McCOY: He [Kirk] means that he feels safer about your guesses than most other people's facts.

All of this leaves me with a question:

Why did Spock say "marshmelon" instead of "marshmallow"?

Was he referencing some archaic or less common form of the word? Is it a 23rd-Century term for it? Did he simply make an error?

(Just to clarify, I am seeking an in-universe reason for how Spock arrived at this particular word. Out-of-universe, the fact that the scene was inserted for mild comedic relief is clear to me.)