On explaining this term to one of my contemporaries, I remarked that there was enough material in it for an encyclopaedia entry. I was consequently asked to write one.
Grace is a word used by Weft and his fellow chattels. It is not an imprecation, though its usage (see below) includes contexts in which other people might use such. It has religious connotations, explained below.
Monks do not use profanity, so this is one of their few permitted options. Weft's proprietors do not appear to object to non-monks using the term.
Any (or, more likely, several) of the following nuances may be meant. Work it out from context or the volume of the screech in which the statement is delivered.
In religous terms, the term is an extension of the general noun. Elegance is believed to be a particular property of (male) gods; thence the concept of 'divine' grace, being a superhuman level of poise and elegance, and so to the specific term meaning "a godlike ideal of skill and beauty to which we inferior and frail mortal monks aspire".
When used in a hopeful sense, the word means "if my [or your, etc] miserable, unworthy abilities are up to it", and similarly with other usages. When used as invocation it can be thought of as short for "divine grant me the grace [to...]", or, as a distancing tactic, "grant me the poise and detachment".
Incidentally, this is one example of the cultish practice of investing ordinary words with a specific meaning not easily comprehensible to outsiders, having the effect of discouraging insiders from associating with those who do not understand them while simultaneously enhancing a feeling of superiority and secret knowledge.
The term is not related to other religions' concept of (undeserved) divine favour or good will, despite being used in similar contexts. The Offwhite City cult does not believe in leaving things to the whim of their various deities. They would say that divine generosity is earned through hard work, and it is only human generosity that has the capacity to be irrational, altruistic or self-serving.