For many years, lawyers were called upon to be "zealous advocates" for their clients. This language has fallen out of favor with the attorney-ethics community, and now no state in the US uses such term. The Texas Disciplinary Rules of Professional Conduct (and is that title so lawyerly or what? Why use two or three words when you can use more?) requires "competent and diligent" representation, although it does use some more high-flown language in its preamble, where both "zealous" and "advocate" appear - just not next to one another.
But what did it mean to be a "zealous advocate?" Why did that term motivate attorneys for over 200 years? I dug out the OED (and accompanying magnifying glass) and looked for the word etymologies and definitions to see if I could find a clue:
advocate ... [a. OFr. avocat, ad. L. advocatus, one summoned or 'called to' another, esp. one called in to aid one's cause in a court of justice; prop. pa. pple. of advoca-re, f. ad. to + vocaire to call. ... lit. one called in, or liable to be called upon, to defend or speak for. ... 2. fig. and gen. One who pleads, intercedes, or speaks for, or in behalf of, another; a pleader, intercessor, defender. ... b. Specially, applied to Christ as the Intercessor for our sins.
zeal ... 2. In a specialized sense: Ardent love or affection; fervent devotion or attachment to (a person or a thing).
zealous ... 1. Full of or incited by zeal; characterized by zeal or passionate ardour; feverently devoted to the promotion of some person or cause; actively enthusiastic.
So, in essence, attorneys are ones who are called to be a passionate and enthusiastic voice for their clients, to plead for the promotion of their cause with fervor, and to (how odd) have an attachment and affection for their client and their client's interests.
Boiled down, we speak for them. We are called to be their voice. In representing them, we act not on our own behalf but always on theirs - we give our voice to them and their interests, never to our own. All of which sounds fairly easy when you think of the classic lawyer example in fiction: Atticus Finch, who gives a voice to a man who has none in society. But none of which sounds like what we do each day when we walk into the office each morning and pick up where we left off the night before - and that office is just one of thousands in the sea of minnows that make up a large corporation.
It is hard to square being a zealous advocate with working for a large company, full of people with competing interests. It is confusing sometimes to sort out the true interest of our company as the client, and make that our goal - shutting out all the competing voices in an effort to remember whose voice we are called to be and on whose behalf we are really interceding. Nevertheless, it is a calling, and a noble one at that. Being given the opportunity to be a voice for another is no small responsibility; may we all fulfill that responsiblity in a way that would make Atticus proud.
Postscript: Many thanks to the Livesays, who used this "advocate" definition in another context and got the wheels of my brain turning on the subject.