Common legal phrases and what they mean

Bona fide
Implies sincere, good intention regardless of outcome/
Per capita
By the head. 
When used by itself, refers to a qualification, or a warning.
Per se
By itself. Something that is, as a matter of law.
Compos mentis
Of sound mind.
Persona non grata
Unwelcome person. 
De facto
Often used to mean something that is true in practice, but has not been officially instituted or endorsed. "For all intents and purposes".
Prima facie
A matter that appears to be sufficiently based in the evidence as to be considered true, "on the face of it".
De jure
Something established in law, whether or not it is true in general practice.
Pro bono
For the public good. Professional work done for free. 
Having been made in error.
Pro forma
As a matter of form. 
An intimation about someone or something, made indirectly or vaguely suggesting the thing being implied. Often used when the implied thing is negative or derogatory.
Pro rata
From the rate. A calculation adjusted based on a proportional value relevant to the calculation.
Quid pro quo
This for that. An equal exchange of goods or services, or of money (or other consideration of equal value) for some goods or services. 
Ipso facto
Used in the context that one event is a direct and immediate consequence of another. "In and of itself".
Status Quo
The "state in which before" or "the state of affairs that existed previously".
Modus Operandi
Manner of operation. A person's particular way of doing things. Used when using behavioural analysis while investigating a crime. Often abbreviated "M.O."
Under penalty. A writ compelling testimony, the production of evidence, or some other action, under penalty for failure to do so.
Vice versa
The other way around. Something that is the same either way.