xmodmap: Changing keyboard map for meta / option on a Mac? Using xterm

How can I remap these keys so they work as Meta?

I'm trying to use Meta-b and Meta-f to go backwards and forwards using emacs mode in my shells. I'm noticing a number of non ASCII characters when I press these Option-b or Option-f.

I noticed in Mac's Terminal.app the option "Use Option as Meta key" checked resolves this issue. It doesn't resolve the issue in my "xterm" session though. I think I can remap Option globally in Preferences, however this would impact all Mac applications.

I see some posts referencing setxkbmap and xmodmap. I tried xmodmap per [this][1] post and keys are still not working.

$ cat ~/.xmodmap

clear Mod1
! keycodes 66 and 69 are the option/alt keys; assign them as Meta keys
keycode 66=Meta_L
keycode 69=Meta_R
! now tell X11 that the Meta keys act as the Mod1 (meta) modier key; that is, when they are pressed with KEY, it is the same as hitting M-KEY
add Mod1 = Meta_L Meta_R

$ xmodmap ~/.xmodmap

 X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  14
  Current serial number in output stream:  14

The keys output when I press OPTION-f after xmodmap:

$ æ

Pressing left option, shows mapping but it doesn't impact the shell behavior:

$ xev

KeyPress event, serial 34, synthetic NO, window 0xa00001,
    root 0x141, subw 0x0, time 613883038, (791,832), root:(791,855),
    state 0x0, keycode 66 (keysym 0xffe7, Meta_L), same_screen YES,
    XKeysymToKeycode returns keycode: 19
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False