What does the notation $(1^-,2^-)$ mean in the ground-state nuclear spin of lithium $10$? I am trying to get to grips with nuclear spin. However I note that many of the spin of isotopes can have many results. For example, if one looks on the Wikipedia page for the isotopes of lithium, the spin of $^{10}$Li, ground state, is quoted as $(1^-,2^-)$. And it says 

The values of spin with weak assignment arguments are enclosed in parentheses.

So what does that mean, we don't know what the spin is? Does $^{10}$Li have two spins?
 A: This seems to be pretty standard notation. The nuclide charts for the IAEA Nuclear Data Services, KAERI and the National Nuclear Data Center all give the $J^P$ of $^{10}\mathrm{Li}$ as $(1^-,2^-)$, brackets and all. For example, the NNDC provides a list of levels for this nuclide which does provide spin information for other levels, but again the bracketed notation for the ground state of the nuclide.
I think the clearest explanation of this notation is in the NNDC glossary:

$J\pi \quad\quad\ $Meaning
(1,2+) $\quad$     We think that the spin and parity are either 1+ or 1- or 2+, but we are not 100% sure

This is backed up by the IAEA NDS guide to the tables, which reads

$J^\pi$ Angular momentum and parity of the state. Values between round brackets are uncertain (based on weak arguments, see the ENSDF manual pag. 103), values between square brakets are assumed from theory.

In essence: there simply isn't enough information available on the spin of this nucleus to tell whether its ground state has $J=1$ or $J=2$.
