It seems to have been adapted from an English legal phrase, but it doesn't necessarily make perfect sense:
The curious obscurity surrounding the origins of the Department's seal makes it difficult definitively to interpret the motto appearing on it. As I suggested above, no evidence has been unearthed that indicates unambiguously how, why, or when, the Department's motto was chosen and placed on the seal, or what its exact meaning may be. According to a longstanding (and officially-sanctioned)19/ Department tradition, however, the motto
was suggested to Attorney-General Black by a passage in Lord Coke's Institutes, Part 3, folio 79, which reads thus:
And I well remember, when the Lord Treasurer Burleigh told Queen Elizabeth, Madam, here is your Attorney-General (I being sent for ) qui pro domina regina sequitur, she said she would have the records altered; for it should be attornatus generalis (i.e., (your) attorney general;") qui pro dominal veritate sequitur.
The first of these phrases is believed to have been quoted by Burleigh from a Latin form then in use (all judicial proceedings were at that time required to be recorded in Latin) in making up the record of actions brought by the Attorney-General on behalf of the Crown. It is translated, "who (the Attorney-General) sues for (or on behalf of) our lady the Queen." "Sequor" is employed in the same sense (i.e., to sue or bring suit) in the Statute of Westminster 2, Chap. 18, as follows: "in elections illius qui sequitur pro hujusmodi debito" (see Coke's Institutes, Part 2, folio 394). In fact our word "sue" comes from "sequor" (See Century Dictionary).<20/>
Dean Pound elaborated upon this story and offered his explanation of the motto thus:
The matter is very simple indeed. The "pro" goes with the noun and the verb. The motto is taken from the commencement of a pleading in a proceeding by the Attorney-General at common law. ...(U)ntil the reign of George the Second, all pleadings were in Latin. The Attorney-General began, "Now comes so and so, Attorney-General, who prosecutes on behalf of our Lord, the King." In the reign of Elizabeth, of course, this would have been "who prosecutes on behalf of our Lady, the Queen." Domina Justitia - our Lady Justice<21/> - was substituted for our Lady the Queen, or our Lord the King. In other words, the seal asserts that the Attorney-General prosecutes on behalf of justice. This would seem a very appropriate motto for the Federal Department of Justice.
I remember reading Mr. Easby-Smith's account of this and it seemed to me very baffling on this point. The passage in Coke's Third Institute
means that when the Lord Treasurer introduced Coke as Attorney-General to Queen Elizabeth he said in Latin, "Here is your Attorney-General qui pro domina regina sequitur", (sic) that is, who prosecutes for our Lady the Queen<.> Elizabeth, who was an excellent scholar, answered, "It should be, Attorney-General who prosecutes for our Lady the Truth."<22/>
Other, basically similar, interpretations of the motto - some grammatically suspect, others more or less literal than the foregoing, but none inappropriate to the Department's mission - have been advanced.23/ Notwithstanding such alternative translations, however, following Dean Pound and the Department's immemorial tradition, the most authoritative Department opinion 24/ suggests that the motto refers to the Attorney General (and thus to the Department of Justice), "who prosecutes on behalf of justice (or the Lady Justice)."25
http://www.usdoj.gov/jmd/ls/dojseal.htm