To be 'lightly toasted' is to be stoned due to the use of marijuana.
This phrase was apparently popular in some circles in the early
1990's... according to this philosophy professor I had
this one time.
Advantages of the phrase 'lightly toasted' over other more common terms:
1 - It is basically a thinly
veiled version of the more popular adjective 'baked,' and yet it is somehow ambiguous enough to be used in virtually any environment without anyone being the wiser.
2 - It invokes images of lightly toasted food, which is always pleasurable to think about and even better - to consume when you yourself are 'lightly toasted.'
3 - You can insert the phrase into a sentence about food and in so doing tip off only those cool enough to understand.