| furtive(obscure, strange, unknown, clandestine, underground) |
| gasser(adjective, hard, fancy, difficult, complex) |
| grave(sound, intense, heavy, fierce, knowledgeable) |
| grounded(knowledgeable, cultured, educated, erudite, enlightened) |
| hairsplitting(sharp, obscure, exact, precise, discriminating) |
| hairy |
| hard-earned |
| hard-fought |
| hermetical(obscure, dark, secret, unknown, hidden) |
| hi-tech(adjective, hard, fancy, difficult, complex) |
| hid |
| high-flown(abstract, theoretical, philosophical) |
| high-tech(adjective, difficult, complex, special, sophisticated) |
| highbrow(adjective, obscure, knowledgeable, cultured, educated) |
| immaterial(intellectual, abstract, insubstantial, theoretical, imperceptible) |
| impalpable(abstract, insubstantial, theoretical, imperceptible, philosophical) |
| imperceivable(obscure, dark, secret, unknown, hidden) |
| in a cloud(expression, idiom) |
| in a fog(expression, idiom) |
| in eclipse |