I've noticed that sometimes loan words (usually pairs) are truncated to just the first two morae from each word:
- Smash Brothers ➤ スマッシュブラザーズ ➤ スマブラ
- "Don't mind." ➤ ドンマイ
I assume this is done to shorten the writing and pronunciation. Is the accepted practice to just grab the first two morae from each word and shove them all together?
It depends on the words and how they are pronounced, although the pattern you noticed is common.
For example, the following don't fit the pattern you see:
Brad Pitt -> ブラピ
Ice Cream -> アイス
Convenience Store -> コンビニ
There has been a lot of linguistic studies about this and many patterns that exist. One rule is that they are always truncated to 2 to 4 mora. Also, [濁音]{だくおん} are avoided if possible and generally heavy syllables are avoided at the end.
Here is a link to one paper if you are interested.
It is common to abbreviate a loanword to four morae, and when the original loanword is a compound word consisting of two components, then it is common to do so by taking the first two morae from each component. Sometimes sokuon (little tsu: ッ) and chōon (ー) are skipped when counting two morae for this purpose.
Sometimes the final sokuon or chōon after abbreviating is dropped, and this explains ブラピ in Jesse Good’s answer and ハピバ in Tim’s comment on Jesse Good’s answer.
But not every loanword is abbreviated like this, as Jesse Good already wrote.
This pattern is not really specific to loanwords. I have seen this pattern applied to the title of comics, video games, and TV programs. In some sense, even abbreviations such as [外国]{がいこく}[為替]{かわせ} to [外為]{がいため} may be viewed as an instance of the same pattern.
By the way, I do not think that ドンマイ is an abbreviation of something like ドントマインド. It seems to me more like a transcription of the pronunciation of the phrase “Don’t mind” heard by Japanese speakers.
