kanji - How to classify the non-standard reading of "お土産{みやげ}"?

Is お土産{みやげ} an 当{あ}て字{じ}、 熟字訓{じゅくじくん}、 or something else?

For 「お土産」 to be 当て字, the「み」reading must be part of the 音{おん}、訓{くん} readings or a 名乗り{なのり} reading for the「土」kanji. This is not the case.

For 「お土産」 to be 「熟字訓」, the meaning of the characters 「土」and「産」when placed side-by-side must approximate the meaning of "souvenir". And this is not the case.

So, is the reading of 「お土産」 classified as "non-standard", but not in the sense of 「当て字」 or「熟字訓」? Maybe there is a 3rd classification for words with non-standard readings? Is this 3rd classification maybe "難読{なんよ}み" (I've found very little info about 難読み so far).

Answer

I think you might be getting deceived by the English word "souvenir" in thinking 土 + 産 has no meaning connection to みやげ. The English word tends to mean something you buy for yourself to remember your travel. The Japanese word is for things you buy to give to others that reflect the cuisine of where you travelled.

産 means either to birth a child or to produce goods or the goods necessary for life.

And 土 means earth, dirt, and some other things but particularly relevant is that it means 地方 ("geographic area" but much more colloquially used than the heady-sounding English equvalent). [Thanks snailboat for the improvement!]

Seems like a 熟字訓 to me. Moreover, the Japanese Wikipedia specifically lists it as one stating:

その土地の特産品、旅先で仕入れた品物、記念品。土産物（みやげもの/どさんぶつ）。