A woman should not appear in front of men wearing pants or trousers for two reasons:
1 – They show the shape of the woman’s legs
2 – Wearing them is an imitation of men
Shaykh Ibn ‘Uthaymeen said, What I think is that it is haraam for women to wear pants, because they are an imitation of men. The Prophet (peace and blessings of Allaah be upon him) cursed women who imitate men. It also takes modesty away from the woman, because it opens the door to wearing the clothes of the people of Hell, as the Prophet (peace and blessings of Allaah be upon him) said: “There are two types of the people of Hell whom I have not seen…” and he described one of them as being, “women who are clothed yet naked, walking with an enticing gait, with something on their heads that looks like the humps of camels, leaning to one side. They will never enter Paradise or even smell its fragrance, although its fragrance can be detected from such and such a distance.”
Majmoo’ Fataawa: 12, question no. 192, 104.
With regard to wearing pants under a jilbaab, there is nothing wrong with that, rather it is more covering and more protective, if the jilbaab is wide and concealing and does not have any splits that show what is underneath it.
Shaykh ‘Abd al-Razzaaq ‘Afeefi (may Allaah have mercy on him) said: If a woman wears pants and puts over them a wide garment then she will not be resembling men, so long as she wears them underneath her garments. End quote from Fataawa al-Shaykh ‘Abd al-Razzaaq ‘Afeefi, p. 573.
The basic principle is that the woman’s jilbab should be wide and cover the tops of her feet, because of the report narrated by al-Tirmidhi (1731), al-Nasaa’i (5336), Abu Dawood (4117) and Ibn Maajah (3580) from Ibn ‘Umar (may Allaah be pleased with him) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever lets his garment drag out of pride, Allaah will not look at him on the Day of Resurrection.” Umm Salamah said: “What should women do with their hems?” He said: “Let them down by a handspan.” She said: “But then their feet will be uncovered.” He said: “Then a cubit, but no more than that.” This hadeeth was classed as saheeh by al-Albaani in Saheeh Sunan al-Tirmidhi.
(See Islam Q&A Fatwa: 60131)