Ar is usci loch aelta…
Ar is usci loch aelta .i. ni maith do dig, ní maith do indlut.
(for * tis * water * of-lake* limey * i.e. * is-not * good * for * drink * not-good * for * bathing)
For it is an alkaline lake: not good to drink and not good for bathing.
In other words, it is a useless thing, person, or situation. In this case, Saint Brigit is telling a clerical student that a person without a spiritual guide is of no use. Her full statement, which is found in a short anecdote in the Book of Leinster (MS folio 283b28), is:
"Air is coland cen chend duine cen anmcharait. Ar is usci loch aelta .i. ni maith do dig ní maith do indlut. is cumma & duine cen anmcharait."
"For a a person without an soul-friend (anamchara) is a body without a head. For he is an alkaline lake, not good for drinking and not good for bathing, like a person without a soul-friend."