An uncommon Japanese verb base. The only context in which you are likely to encounter or make use of it is the "informal inclusive command form", which is a fancy of way of saying "the let's do x form". Less polite than the -mashoo form used with Base 2.

To form Godan verbs into Base 5, follow this pattern:

For Ichidan verbs, drop the final -ru and replace it with yoo. For example:

Kuru and suru become koyoo and shiyoo in Base 5.

As described above, Base 5 with no ending is the inclusive imperative:

Let's go home.

Asu mata koyoo ka.
Shall we come again tomorrow?

Another possible ending is to omou, which conveys "I think I'll...". Note that omou is a verb itself and must needs be conjugated:

Watashi wa atarashii kuruma o kaoo to omoimasu.
I'm thinking of buying a new car.