AsciiMath doesn't have a command for obtaining bold-italic letters, but you can add one by inserting
<script type="text/x-mathjax-config">
MathJax.Hub.Register.StartupHook("AsciiMath Jax Config",function () {
AM.symbols.push(
{input:"mathbi", tag:"mstyle", atname:"mathvariant", atval:"bold-italic",
output:"mathbi", tex:null, ttype:AM.TOKEN.UNARY}
);
});
</script>
to your HTML file somewhere before the script that loads MathJax.js itself. Then you can use `mathbf(m)` to obtain the bold-italic "m" that you are looking for.
Davide