What words or phrases are there which you use (or used to use) but wouldn't be widely understood? They may be dialect words which are peculiar to the region in which you live, or they may have been coined by your family or friends. Existing English words count, if they have a special meaning when used by your family.
My five are as follows:
Mardy. adj. Commonly used by children in Leicestershire to mean something along the lines of "moody" or "sulky". Often combined with other words e.g. "mardy-bum," "mardy-boots," as in "Don't be such a mardy-boots".
Link-link chains. n. Paper chains. (Don't laugh!) They were called this in a book which I had when I was very small, which featured easy craft projects for kids, and the name just stuck. I have to make a conscious effort to remember to call them "paper chains" when adults are listening.
Grass-den. n. (I'm only putting this in because I wonder if it was specific to one of my primary schools -- nobody else seems to have come across the concept, let alone the term!) When the playing field had been mowed, there were naturally heaps of grass-cuttings all over it. We used to gather these up and create "grass-dens", which were effectively plans for buildings laid out in grass. We'd create the outlines of walls and doors in mounds of grass at most a few inches high, and these would be our "den" for our "gang" (I'm assuming these terms at least are in slightly more common use!). We'd protect them fiercely, while trying to steal heaps of grass from other people's dens; what I find interesting is that the walls functioned as very real boundaries even though they were easily steppable-over or kickable-over.
IDST abbr. The acronym expands as "If destroyed, still true", and may be appended to a graffito or blackboard-chalkage to ensure that the libel (e.g. "Simon Parry-Jones smells") or declaration of allegiance (e.g. "JM 4 AW") remains "true" even if obliterated by the libelled party (or of course by blackboard-cleaning and desk-graffiti-removing figures of authority). This was later extended to "IDOAST", that is, "If destroyed or altered, still true", after some people who presumably went on to become lawyers argued that by changing "Laura Carrier is a cry-baby" to "Laura Carrier is not a cry-baby" the original statement could be falsified, having not been destroyed and therefore having failed to invoke the IDST clause. (I find this little snippet of playground voodoo absolutely fascinating because of the way it recognises yet simultaneously undermines the power of the written word. Perhaps I'm over-analysing it...)
Abba-dubba n. 1. A letter formed by placing a lower-case "d" and a lower-case "b" back-to-back, so that they share one upright. 2. Father Christmas [etym. obscure]. I came up with this word when I was less than 2 years old. As used to refer to the hieroglyph described, it may be a corruption of "A, B, D, B"; all I know is that I invented the letter by putting the relevant fridge-magnets back to back, and proudly announced that that was what it was. But I have no idea why it refers to Father Christmas; I suspect there's some complicated ideographic/phonetic process going on rather like that involved in the development of Japanese kanji. The use of "Abba" for "Father" is also interesting... no, it's not really, is it. I suspect the most plausible explanation is that, like most toddlers, I spent a lot of my time spouting nonsense syllables just for the fun of playing with the sounds. :-) Needless to say, the word is now more or less obs., though I don't think I'm quite old enough for it to be arch. yet.