""Well, I have been obliged to reconsider it. I had my net drawn tightly round Mr. Sholto, sir, when pop he went through a hole in the middle of it. He was able to prove an alibi which could not be shaken. From the time that he left his brother's room he was never out of sight of someone or other. So it could not be he who climbed over roofs and through trapdoors. It's a very dark case, and my professional credit is at stake. I should be very glad of a little assistance."

The derivation of 'alibi' seems worth checking.From AHD:
SYLLABICATION: al·i·bi
PRONUNCIATION: l-b
NOUN: Inflected forms: pl. al·i·bis
1. Law a. A form of defense whereby a defendant attempts to prove that he or she was elsewhere when the crime in question was committed. b. The fact of having been elsewhere when a crime in question was committed. 2. Usage Problem An explanation offered to avoid blame or justify action; an excuse.
VERB: Inflected forms: al·i·bied, al·i·bi·ing, al·i·bis
Usage Problem
INTRANSITIVE VERB: To make an excuse for oneself.
TRANSITIVE VERB: To make an excuse for (another).
ETYMOLOGY: Latin, elsewhere, from alius, other (on the model of ibi, there). See al-1 in Appendix I.
USAGE NOTE: When used as a noun, alibi in its nonlegal sense of “an excuse” is acceptable in written usage to almost half of the Usage Panel. As a verb (they never alibi), it is unacceptable in written usage to a large majority of the Panel.

From the Latin, meaning elsewhere.