lots to comment on here, but let me start with the word "tip."
TIP is almost certainly not an acronym. very few pre-20th century words are acronyms and "tip" dates back to the 1600s. and, yes, if it were an acronym, it would in fact be "tep," because you are indeed attempting to "ensure" a level of service, not "insure" it.
but, as pointed out, that makes little sense because tips are almost always given out after service has been provided (with apologies to sthrngary, who is know to "tip" butlers up front).
the word "tip" was used by thieves and beggars as part of a secret language they developed so that others wouldn't know what they were talking about. its original meaning was "to give or to share." over the next hundred years or so, the word somehow transformed to its modern meaning.
https://www.snopes.com/fact-check/tip-sheet/
with regard to the comment that we must remember that - for most butlers - english is a second language... that may be true, but that is probably true of 90% of the ship's crew. in many cases, and particularly in the case of filipino workers, they are chosen specifically because of their english language proficiency. it's the same reason the philippines has taken over as the number one country for outsourced contact centers. in my experience, most NCL butlers are filipino.
i have spent an extraordinary amount of time in the philippines on business and the folks are very customer service oriented and they will do anything they are asked.... but the request must be explicitly stated. any misunderstandings are usually due to cultural upbringing and not language gaffes. just as with a restaurant server, it's a two way street. the guest must first properly state the request. (this is, perhaps, why gary's recommendations to give a letter or printed list to your butler works so well.)
with regard to a butler's repeated reminders to tip... that's completely inappropriate, in my opinion. the reason it's different than a card on the desk or an envelope with a staff member's name on it is because those things are passive, whereas the butler's comments are aggressive.