  Berwick-Evens and Lerman recently showed that the category of vector fields
on a geometric stack has the structure of a Lie $2$-algebra. Motivated by this
work, we present a construction of graded weak Lie $2$-algebras associated with
quasi-Poisson groupoids based on the space of multiplicative forms on the
groupoid and differential forms on the base manifold. We also establish a
morphism between the Lie $2$-algebra of multiplicative multivector fields and
the weak Lie $2$-algebra of multiplicative forms, allowing us to compare and
relate different aspects of Lie $2$-algebra theory within the context of
quasi-Poisson geometry. As an infinitesimal analogy, we explicitly determine
the associated weak Lie $2$-algebra structure of IM $1$-forms along with
differential $1$-forms on the base manifold for any quasi-Lie bialgebroid.
