can any one tell me how and when the Scottish dirk originated and how it became part of the highland dress?

It dates back as far as the bronze age and was originally used for hunting and skinning. It became part of everyday dress, because it dates from a time when people had minimal posessions and therefore kept the most precious ones on them at all times. Worked metal - such as a dirk - in the bronze age would qualify as such. Additionally, in a time when people were primarily occupied with finding sustenance it would make sense to keep a hunting knife on your person at all times so you'd be ready whenever you'd happen to come across prey.

The modern dirk, which is of a more military purpose, was developed in the early medieval period. It was designed to be able to punch through metal plate armour and stab someone in their vital abdominal organs, and for this reason is also sometimes called a punching dagger or a kidney dagger.

Even after its primary purpose shifted, the dirk frequently did double-duty as both a hunting and martial tool as the common person likely couldn't afford a variety of special knives. Even if they could, it would be terribly inconvenient to carry more than you absolutely had to when living on a battlefield.