Possible Clues:

• Tough to budge
• A need to rehash the leftover - anything but a soufflé!
• Like the sky when the weather's heavy?
• Lacking lightness
• Heavy and slow, as if weighted down with heavy metal
• Heavy or slow, like dark-gray metal
• Overcast
• Field study that's dull
• Like a winter sky
• Oppressively sluggish